text:
Why can't Centre of pressure and Centre of Gravity coincide?
I understand that normally CG is placed ahead of COP so that a tail down force can make the aircraft stable. But why can't the COP and CG be placed in the same position so that there needn't be a tail down force to stabilize the aircraft.
I read an answer in Quora saying that "When the CG and CP coincides, the pitching moment will be 0 as the distance between them is 0" What does that mean?

Their lengthwise position must coincide for straight flight, if we speak about the CoP of the whole aircraft. It is the neutral point that has to be aft of the center of gravity.

Interestingly, non-scale rubber powered free-flight aircraft designed for endurance almost always have a tail designed for upforce rather than downforce. What's required for stability is just that the tail have less lift than the wings (lower loading) - this does not necessarily mean downforce only less upforce. Also interestingly, this does still result in overall CP being behind CG (the CP of the wings may be at or ahead of CG but the tail now also provides lift so the aircraft's CP is still behind CG). I often "fly" planes with CG being behind the trailing edge of the wings!

Centre of pressure (CP), by definition, is the point at which the aerodynamic moment of something is zero.
If you are talking about the CP of the whole airplane, then at trim condition, the CG must always coincide with its CP. Period. This is achieved for any operational speed through well designed pitch control that varies in flight. For example, stabilizer/elevator for tailed airplanes, elevons for flying wing.
If you are talking about the CP of a single aerodynamic surface, like a wing or tail, then it gets more nuanced. Let's restrict the discussion to pitching moment only. First of all, CP is not a very useful measure of stability: it always moves around for a cambered airfoil. In fact, at an AOA where the lift is zero for a positively cambered airfoil, it moves to infinity!
Instead, use aerodynamic centre (AC) for stability analysis. For pitch stability, you would like the CG to be ahead of the AC of the entire airplane (this is called the neutral point).

Because:

If centres of pressure and of gravity would coincide, there would be no inbuilt tendency for the nose to drop upon an adverse aerodynamic event like a stall.
If someone walks along the aeroplane and shifts the CoG, the two centres no longer coincide. There is some range required for the CoG.

An aeroplane must have static and dynamic stability in order to be easy to control: with a gust hitting the aeroplane, a statically stable aeroplane returns to a stable attitude without the pilot having to apply a correction. Like a car driving straight when not holding the steering wheel. This answer explains why.

The image is from the referenced answer as well, and shows three centres of pressure: for the main wing $(C_{Nw})$, for the tailplane $(C_{Nh})$, and for the whole aeroplane $(C_{N})$. Note that in cruise, it is no issue if the resulting centre of pressure $(C_{N})$ is on top of the CoG.

Adding to your point about the CoG lying in some range:  Even if the passengers stay in their seats, the CoG can also shift as the fuel is consumed.

Your picture shows something different to what you are trying to explain. It shows aerodynamic centres (a.c.) rather than centres of pressure; this is why the 'forces' say $\mathrm{d}C_N$: they are derivatives by AoA, not the forces per se. Only by using derivatives (and the concept of a.c.) can you explain static stability; CP has almost nothing to do with it. In fact, in a straight flight, it will coincide with CG; there will be an 'issue' if it doesn't.

"Upon an adverse aerodynamic event" the CP (which previously coincided with CG) simply moves and thereby provides the "inbuilt tendency" to restore the original conditions. To make the airplane stable, we just need to ensure that it moves in the right direction.

It's because normally tailed aircraft (and flying wings) depend on an equilibrium state between opposing pitching forces for static stability, and an inherent tendency of the aircraft to seek this equilibrium state on its own.
So you need a nose down pitching moment that is always present, acting about some "balance point", being the aerodynamic Neutral Point, which is wing CP +/- all other pitching forces also present, like thrust line forces, other lifting forces (like MAX nacelles), drag from things sticking up or down (like floats), etc. ending of at some net location for any given configuration, loading and flight condition.  Although a gross oversimplification, you could think of the Neutral Point as a seesaw pivot (although a dynamic gaseous one) for the purpose of imagining it in your mind.
The nose down pitching moment (you at one end of the seesaw) is balanced about the Neutral Point by an equal moment applied at the tail - the tail's downward lift (someone at the other end of the seesaw).  The balance of forces for a given configuration will be achieved at some angle-of-attack/speed.  At this point, you are "in trim" and the airplane will not want to accelerate or decelerate and will hold a constant AOA/speed/pitch attitude all by itself.  It's statically stable.
Move the CG forward and the nose down pitching moment increases, and this requires an opposing increase in tail down force.  The tendency to seek an equilibrium state is increased and static pitch stability improves, so static stability is highest at the forward CG limit.  Move the CG aft toward the Neutral Point and ND pitching moment decreases (you're moving closer to the seesaw pivot), and tail downforce required for balance decreases, until with CG at the Neutral Point there is no nose down pitching moment and no tail downforce required to oppose it.  Whatever equilibrium state there is is very weak or non-existent with the tail largely unloaded, and the airplane will pitch randomly and won't settle down at any AOA/speed/attitude.
Move the CG any farther aft, and now the tail has to start to lift UP, and it all goes to hell and the airplane becomes statically unstable, and eventually uncontrollable.
So, going to the seesaw analogy again (yes it's way too crude but it works for the purpose of visualization for a layperson), you need to be on one end of the seesaw with someone pushing down on the other end, with a force you can control from your end.  When you move the CG aft to the neutral point, you've shifted your body to the pivot of the seesaw (CG and CP/NP coincides, as per your question), the person pushing down is no longer needed, and there you are trying to balance it like you're on a tight rope as it wants to tip this way and that by itself in response to the slightest disturbance. The airplane is neutrally stable and is a handful to fly unless it has an active artificial stability system to operate the tail surfaces to mimic natural static stability.
If you've flown an airplane a lot at the maximum aft limit (I did it a lot bush flying) you can really sense this. You still have positive static pitch stability but it's noticeably weaker than with a more forward CG and the airplane can be unpleasant to fly especially in bumpy air, although not dangerously so.


text:
The 2 person cockpit rule - what happened?
I was recently on an EasyJet flight and was chatting with the FO during the cruise about flying Microlights and other aviation fun. When he went back to the flight deck, I noticed that the Captain had been left in there alone.
I asked the cabin crew manager about it and she explained that the 2 person cockpit rule was abolished some time ago (she couldn't remember when) and that the door is now opened via facial recognition (I did notice the FO look up at a camera to unlock the cockpit door).
I then asked her if the door could still be locked from the inside - she confirmed it can.
So my question is - how does this prevent another Germanwings? I asked her this question and she agreed that really... it doesn't.

Ppl have to be able to go for a dump. What else are ya gonna do?

@JohnK cabin crew in the flight deck until pilot returns

The problem was "cabin crew in the cockpit until P2 returns" had other safety/security implications

If I really want to kill everybody, all I have to do is shove the column forward on final when we are at 200 ft.  That's it, done. Absolutely nothing the other pilot can do about it.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
How big would a single propeller need to be on a 737?
If you wanted a 737 to have the same design as a single prop light aircraft (i.e. just one prop on the nose, no turbines, APUs or other engines) - how big (and how fast) would the propeller have to be to provide enough thrust to provide lift and enable flight?

Just a note on the OP’s question: if you have a single propeller - no turbines, APUs or anything else to drive it - what you have is a speedbrake.

@Romeo_4808N I'm talking about if it was a large 912UL or similar

Obviously it's hypothetical, so we don't need to be concerned about what happens during takeoff and landing (probably history's greatest prop strike). But it might help to clarify if you are referring to just staying aloft in level flight at minimum speed, or flying at normal turboprop speed and altitude.

As usual, we can use the simple momentum theory to quickly estimate the thrust $T$ generated by a propeller:
$ T = \sqrt[3]{2 k \rho A P^2} $
where $A$ is the propeller area, $\rho$ is the density and $P$ the power needed to spin the propeller. This equation is strictly valid only at zero forward speed but being the highest thrust normally required at takeoff, then it's just perfect for our purpose. $k$ is a coefficient taking into account the limitations of the momentum theory: a value of $0.5$ can be used in a conservative way.
So, the two CFM LEAP-1B of a 737 Max generate a total thrust $T$ of 260 kN. Substituting we get:
$AP²=\frac{T³}{2k\rho}=\frac{260'000³}{2 \times 0.5 \times 1.125}=1.5e16 [m²W²]$
Note that we can only estimate the product of the propeller area with the needed power. We can now decide to either use a very powerful engine with a small propeller or a less thirsty engine and a big propeller.
Taking into account the height of the nose (some 3m) as limit for the propeller radius, we get a propeller area of $A=\pi 3²=28m²$ and therefore a needed power of $P=\sqrt{1.5e16/28}=24MW$. Three of these would be just perfect to power the propeller.

Thrust requirement for a twin engine is different than for a single engine. A twin will need to do with either engine down most of the stuff a single can do with it's only engine, so for this single prop 737 we do not need to match the full rated power of both of the regular 737's engines

260 kN thrust is about 58,500 lbs, a bit more than the MTOW of a Chinook, with two 3-blade 60 ft diameter rotors.  So a single (subsonic) rotor, ahem, prop, would be about 90 ft.  Smaller if you add more blades, as with even heavier helicopters.

With a giant prop, more thrust is created at lower airspeeds, less at higher airspeeds than the LEAP.  Takeoff performance is a selling point for turbo props.  >20,000 hp will create 58,500 lbs (260 kN) of thrust at 0.2 Mach, especially if the "giant prop" is more efficient.  The idea was tried here.

It's 23 foot prop swung at 545 RPM.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Where are some of the most challenging airfields in the UK?
I fly Microlights and am used to short strips, I can land at airfields such as Brown Shutters and Clench Common without much strain.
What are some of the most challenging airfields in the UK for light aircraft? I'm thinking the shortest runways with the most obstacles - I would like to improve more!

I'm not completely against list questions on Aviation, but note they have historically been considered bad on the SE Network https://meta.stackexchange.com/questions/139618/are-list-questions-off-topic

And our own previous discussion on list questions: https://aviation.meta.stackexchange.com/questions/3845/what-should-we-do-with-list-questions - As discussed, a single community wiki answer will be enforced here.

Seems like a caveat is needed here that this is only recommended for experienced pilots and that beginners should stick to the big, wide, long, no obstacle runways until they're pretty darn good at those landings... But, IANAP, so what do I know.

@FreeMan It depends what you learn on. I did all of my training on a 350m grass strip... so I'm alright at short runways despite not being very experienced. Jamiec - noted :)

Might I suggest we widen this question to "The world" and have one CW per country....we might see some more interesting info

Do sheep count as obstacles?  You need to phone ahead at Gigha to make sure they're off the runway.  Never flown there myself but I've heard the surface is a bit challenging.

Tangential, but I absolutely love that there are difficult-to-land-at airfields named "Brown Shutters" and "Clench Common" :)

@AakashM haha yep, they're tough. Someone crashed into power lines at brown shutters back in August - https://aviation-safety.net/wikibase/344844

This sort of question is more interesting when we don't narrow it down to one country but have one answer per country. In that spirit, this answer is for:

United Kingdom
Short
There are a number of airfields with very short runways, probably one of the shortest licensed runways is Netherthorpe at 382m (LDA 308m)
Unlicensed you'll have a few to pick from. Grass runway at Gloucestershire Airport is 305m

Challenging
Often quoted as challenging is Barra which is the only airfield situated on a public beach.

Honourable Mention
It's unclear whether this airfield is still open, it's not shown on any guides, but one of the most challenging airfields must be Vallance-By-Ways. It is situated a couple of hundred meters from Gatwick Airport's runway. You can watch a (pretty scary) video of someone landing a microlight there.

I'm sure there are many many more challenging airfields in the UK, but this is a start on the most often quoted ones that I know of. I hope others can/will contribute.


text:
Being sent away from an airfield - could I have asked more questions?
On the weekend I visited an airfield (it IS inside an ATZ) that has a PPR section on their site. I've previously called ahead on the radio for PPR.
Anyway I called them on frequency 10 miles out and they asked if I had PPR. I said no. The response: "Then we can't accept you unless you have a flight safety issue, have a safe onward flight".
And that was that. Could I have asked more questions or tried to get in anyway? I happen to know it's not a very busy airfield.

OOI, does the AIP state that the airfield is "PPR required"? If so, why didn't you comply?

Also, for the record I only know of 1 airfield in the UK which is specifically non PPR and that's Sandown as I'm sure you know.

@Jamiec no excuse - I was in a rush

no judgement whatsoever, but just a point of safety; there is no such thing as "in a rush" in recreational flying. You haven't got to be anywhere, you've no commercial pressure whatsoever. Every single time I've had any incident while flying it can be traced back  to me (thinking) I was in a rush. Take it slower. If time is a pressure, go a different day.

As a tip, the app Wings Taking Fight is useful because it has a detailed map of airports/airfields across the UK and it lists for each whether PPR is required or not, how to submit, etc.

This is one of the oddities of flying in the UK.
In every other part of the world that I've ever flown airports operate on the basis that they sort of expect aeroplanes to come and land there. In the UK we have this slightly bizarre concept of "Prior Permission required" and as almost all small airfields are privately owned, it's like any private property, you need permission to use it - and the owner reserves the right to make their own rules.
There is some logic behind it, albeit slightly obtuse. Even though almost all those airfields have been there since pre-WW2, the locals love to complain about noise. So to keep people happy, the airfields agree to limit the movements. By asking pilots to request permission before coming, they can reject requests if they have reached some quota. In some cases the land use as an airfield is predicated on PPR being in operation.
Lesson learned - always call/fill in the online form prior to departure.

Specific answers:

Could I have asked more questions or tried to get in anyway?

You can always ask, but their land, their rules. It doesn't matter if there was 1 or 100,000 movements that day. Busy or not, it's their choice. Of course, as an uncontrolled airfield they cannot give you permission to land or otherwise. Don't expect a warm welcome if you land there against their wishes.

@Jamiec you wrote "as an uncontrolled airfield they cannot give you permission to land or otherwise." - doesn't PPR mean you can ask for permission to land, and they can (potentially) give it? Did you mean they can't prevent you from landing? Maybe this is a British/American English divergence, but I'm definitely puzzled.

@Spike0xff: The way I interpret it, they cannot issue a landing clearance nor can they deny one, since at an uncontrolled airfield, the very concept of "clearance" does not exist. As the owner of the private property, which just happens to be an airfield, they can, however, control whether or not you are allowed to enter their property and what you do there, including landing your airplane. So, they can deny use of the airfield in their capacity as the owner of the property, but they cannot act as controller of the airfield.

@Spike0xff I meant exactly what Jorg said

This is not something specific to UK: "As all private airports in Belgium, EBTX is PPR" (source), any private or military airfield in the world is potentially subject to PPR.

PPR means exactly that - you have to have prior permission. The procedure for the visiting aircraft to get that is generally described on landing cards or web pages. The airport itself probably has internal procedures for granting permission. The person answering on radio most probably would not be allowed to grant permission. The exact reason for PPR would vary between different airports of course.
In my history, PPR was introduced at the glider airport. Reasoning was that we got some inexperienced powered aircraft pilots coming there as part of their training or for keeping current on number of landings. Mixing unknown powered aircraft in a busy uncontrolled airspace filled with student glider pilots gave us a bit too much of excitement. Hence the requirement to call ahead on the telephone and get the permission. We in the gliders and tow airplanes were aware of the flight paths and radio signaling around the airport but you could not expect that from unknown pilots. In addition we worked hard together with the surrounding farm houses to avoid disturbing people or live stock by carefully selecting flight paths for powered airplanes around the airport. Add in that the condition of the field was not guaranteed - parts of the year definitely unsuitable for typical powered aircrafts.


text:
What is the differences training syllabus for a Microlight rating to an SSEA rating?
Have looked around for this, but struggling to find the answer.
I have an NPPL (UK) license with a Microlight rating. I want to get the SSEA rating so I can fly 4 seat aircraft without basically starting again with a PPL.
How much training is required and what additional theory or practical exams are there if any?

To add an SSEA rating you need to complete a minimum of 3 hours training covering 2 hours slow flight and stalls and 1 hour instrument appreciation.
However, saying this you also need to get familiar with flying the heavier aircraft, i.e. a Eurofox so it normally takes between 8-10 hours.
Also a practical navigation skills test and a general skills test.
In terms of theory, you need to pass the PPL Principles of Flight exam and the PPL Aircraft General exam.

Links to the sources of this info would improve the answer for future visitors

Hey @Jamiec, I got this as an email response from a local flight school


text:
Airshow pilots not making radio calls - best course of action
I recently flew into an air show, where some very large, fast and expensive historical aircraft were being flown. It is important to note that although this was within an ATZ, it was uncontrolled airspace, with a ground-radio facility and no service available.
The approach was fine as they weren't in the circuit when I arrived. However, on departure it was very busy, the air show aircraft were in the circuit taking people up and landing from experience flights.
I taxied to the "holding point" of the runway in use and received the usual "take-off at your own discretion" from the ground operator. To my surprise, one of the historical aircraft was on final and landed. They hadn't made any radio calls including downwind or final.
Of course, I now didn't know if there was another one behind it, so I made a radio call:
"[callsign], holding short of runway xx, ready for departure, but one of the xxx aircraft has just landed without making any radio calls, do we know if anyone else is in the pattern?"
The aircraft in question immediately responded:
"We're always listening in on frequency, you're good to go".
I found this statement odd, what's the use in listening if you're not speaking, I thought.
Anyway, I had a good look around, took off and left without incident, as I departed the airspace, I heard much more radio activity, including pattern calls from the various historical aircraft, I don't know if this was related to my previous announcement.
So onto the question, what is the best course of action at such events, when radio calls are not being made? I am concerned that I've missed something and perhaps these type of aircraft are not expected, or it is not common practice, for them to make GA calls and maybe I sounded like a douche :) I would like to know the etiquette for future fly-ins.

Always file a safety report if you feel safety was violated.

Flying in to an airshow, or a fly-in (or anywhere with a high volume of traffic for that matter) is a complex and dynamic environment. You absolutely need to be on your listening and watching game!
Remember "Take off at your discretion" is not a clearance. What it essentially means is that you have reached the limit of what ATC can clear you for, and you're now responsible to see and avoid.
The fact is, the pleasure flight/display crews should absolutely have been making calls and if they were not, then it is them that was in the wrong not you.
If at any time you think this impacted the safety of your flight I strongly suggest you file a MOR form - which includes "Voluntary Occurance Reporting":

Other safety related information which is perceived by the reporter as an actual or potential hazard to aviation safety

On another note, are you 100% sure that you did not miss a downwind/base/final call by that aircraft? It's easy enough to do and I can tell you we've all done it!

"We're always listening in on frequency, you're good to go"

This tells me that this pilot was well aware that you were ready for departure, and may well have executed a go around had you have taxied out to the runway. Is this the right thing to do? Possibly not, but you have to expect the pilot of a warbird/commercial pleasure flight would have been far more qualified and experienced than you or I. They, for the most part, REALLY know what they're doing.

I don't think it sounds like you did anything wrong here per se. It's all a good learning experience and it sounds like you learnt some new skills that day. Mission accomplished!

Your best course of action is to make your radio calls and talk to the "air boss" (a North American expression describing the person who is supposed to coordinate airplanes at a fly-in or show) per the normal protocols, but realize this is not a professionally "controlled" environment (ATC wise) and assume, for your own survival, nobody knows what they're doing and you're on your own in the chaos.
That means, go through the steps, follow the protocols for comms etc., but assume other aren't and don't take any action until you confirm visually that it's safe. This means you back track after broadcasting your actions and visually confirming nobody is on final, sit at the button facing the approach path while you get ready, then broadcast your actions and turn around and take off when you are sure nobody's coming, and assume airplanes could be randomly crossing your path and have your head outside the cockpit as you leave.
It's still on you to make sure you don't run into anybody or they don't run into you, to the extent you reasonably can. I fly out of an airfield with a lot of glider activity, and this is a basic fact of life; people are making calls in the circuit, but you have to assume someone could be out there that isn't, so don't take anything for granted.


text:
How much air does a 777-300ER's pressure cabin hold?
I've been wondering how much air a 777-300ER's pressurised cabin holds, but, while most of its specifications are fairly easy to find, the amount of air it contains is not.
Does anyone know what volume of air is held by the pressure cabin of an empty 777-300ER?

You need a fuselage station drawing diagram where you can get the distance between the pressure hull's front and rear bulkheads, along with the fuse barrel diameter, and from there it's a simple volume calculation.   I didn't have much luck, but you might get lucky googling it after some hunting around.

As most things in life, this can probably be approximated fairly well through appropriate use of integrals over solids of revolution. sketch the 777 from the side as a straight tube with two pyramids at both ends (you can taper using another function if you want to be very diligent). Then, revolve this shape around its long axis and compute the volume integral. Now, intersect the volume with a plane from the top and bottom, simulating the cargo compartment as well as the area above the pressure vessel, and subtract that volume. Voila!

Cargo compartments are generally pressurized so the volume should be included.

@Jules The cargo hold is pressurized. The flat cabin floor would have to be extremely heavy to be strong enough to be a pressure bulkhead.

I once read it's about 1 metric ton (1000 kg) of air for a large widebody.
Here's a ballpark calculation to verify this.
The 777's cabin altitude is 2430 meter (8000 ft) when it's cruising at max altitude.
Wolfram Alpha tells us that a cubic meter of air weighs 0.96 kg at that pressure.
DimensionInfo shows that the 300ER is 73.9 m long and 5.86 m wide.
For simplicity, consider the 777 to be a perfect hollow cylinder.  The volume of the cylinder is:
$$\pi \cdot \left(\frac{5.86}{2}\right) ^ 2 \cdot 73.9 = 1993\: m^3 $$
So that's about 1900 kg of air for the cylinder.
You have to subtract a large amount for the air lost to non-pressurized volume, tapering of the cylinder, non-circularity of the fuselage, and volume not occupied by air (structures, furniture, insulation, wiring, luggage, passengers).
I'd guess it's somewhere between 1000 and 1500 kg.
Whether people are on board or not has very little influence.  People float or sink in water depending on having air in the lungs or not.  So the density of a human is around that of water: 1000 kg per cubic meter.  [In other words: you could fit 10 people weighing 100 kg in a cubic meter if you pressed them hard enough].  So the density of air is a thousand times lower than that of the passengers.  Three hundred people - weighing 30000 kg combined - displace less than 30 kg of air.

Huh.  That's surprisingly small!

The volume of a cylinder is pi x radius^2 x length, not pi x diameter x length! So it's more like 2000 m^3.

@Bianfable Of course you're right, thanks. I've fixed it.

This can't be right. You're claiming that between 50 and 75% of the hull is non-pressurized and I can't believe that's even close. It sounds to me like you've just fudged your calculations to agree with the value you remembered.

@DavidRicherby He is not claiming 25% (not 75) to 50% of the hull is unpressurized: as explained in the second to last paragraph, you have to subtract all otherwise occupied volume. Also, "about 1 metric ton" is perfectly consistent with 2 tons depending on your definition of "about".

@Bianfable Whoops, yes, I meant to say 25-50% unpressurized. I still don't find that very plausible.

@DavidRicherby: If he's going with a 777-300ER with a full load of passengers, luggage, cargo, etc., that actually doesn't sound like too bad of a first approximation.  On the other hand, my question specified an empty 777-300ER...

@Sean Also it's not very clear what 'empty' means -- does it still have chairs, or a floor, ... ?

@florisla: Yes.


text:
Why are landing-distance and rejected-takeoff certification tests performed without thrust reversers?
All aircraft have to undergo testing to determine how much runway they need in order to land and come to a stop without running out of runway, and how much runway they need in order to be able to safely reject a takeoff.
These tests are conducted:

With full braking on all wheels so equipped (although with the brake disks and pads worn down right to the metal)
With full use of airbrakes/spoilers (if so equipped)
Without using reverse thrust, even if the aircraft is so equipped, despite the fact that, for anything other than a small general-aviation aircraft, in real-world line operations, reverse thrust is essential for a successful landing or RTO unless conditions are damn near perfect.

Given that thrust reversers are indispensable for slowing any larger-than-small aircraft in less-than-optimal conditions (for instance, if the runway is wet or icy, or if you bounce the initial touchdown and lose the next chunk of runway, or if some of the aircraft’s tyres burst during takeoff, causing the brakes on those wheels to work poorly when you reject said takeoff, or if you're landing overweight, or if the speedbrakes can’t be extended because you’re out of fuel and the aircraft’s ram air turbine can’t power more than the basic flight controls, the navball, and the radio, or if you touch down long, or fast, or long and fast, or if you’re making a forced landing and have to take whatever reasonably-flat surface you get), why are aircraft certified without them?  If it’s to make sure that the aircraft can safely land without reversers, why not also run additional certification tests where the reversers aren’t locked out, but, instead, the spoilers stay retracted, or some (or all) of the wheel brakes are nonfunctional?

"reverse thrust is essential for a successful landing or RTO unless conditions are damn near perfect". So how do you land when an engine is lost in flight? You use a longer runway?

@mins: Well, presumably, yes.

That 3rd bullet point is overstated, verging on nonsense. "Essential"? Hardly. Shortens stopping distance, yes. More so on slick runways. But not at all essential.

Would it be better to have numbers with thrust reversers and then forget to deploy them (or failed reversers) or to have numbers without reversers and use them on landing?
The first case will cause the airplane to exceed the published distance while the second case will cause the airplane to safely stop the aircraft before the published distance.
Simply put, a little extra conservatism on these airplanes.
Please use thrust reversers for the rejected takeoff and landing.  If you know you will land long or bounce the landing, go around and try again.  If you know spoilers are inoperative, get numbers without it.  The same for contaminated runways, use appropriate numbers.

If the reason for your RTO is because of a lost engine, would you want to use reverse thrust with only one engine operative? I think not.

Why not.  Nose wheel steering should be sufficient.

@Gerry Reversers only provide about 30% of forward thrust. If asymmetrical forward thrust from an engine failure didn’t exceed your maneuvering ability, then reverse certainly won’t.

@wbeard52: Plus the rudder, at the higher speeds where reverse thrust is more effective (which, conveniently enough, also happen to be the speeds where the rudder is more effective).


text:
Do any airports other than Mataveri restrict the number of airplanes that can fly within their sphere of influence at any one time?
Mataveri International Airport (SCIP), located on Rapa Nui (Easter Island; Isla de Pascua) in far southeastern Polynesia, somewhat famously, bans more than a single airplane from flying within the area of airspace that is closer to Mataveri than to any other airport (what we might call Mataveri’s “sphere of influence”) at any one time.
This is because:

Mataveri is the most isolated airport on Earth, such that a smaller airplane could easily find itself near the limits of its endurance if it were to be flown there and then forced to divert.
Mataveri has only one runway, meaning that, if said runway were blocked by a crashed or otherwise disabled aircraft, the airport would have to be closed until the runway could be cleared.

My question is: do any airports other than Mataveri restrict the number of airplanes that can be within their sphere of influence simultaneously?

Could you add a reference to the relevant regulation? (Perhaps from the Chilean AIP)

Also I feel like there should be a better term than "sphere of influence" for this concept - perhaps something related to the idea of Voronoi partitioning

This may not be exactly what you're thinking of, but in the US there's a "one in, one out rule" that any towered airport might use depending on operational conditions and needs. There are a lot of possible variables.

Such restrictions are not uncommon. In France rotorcraft operations are limited on some aerodromes to stop excessive noise nuisance in densely populated areas. The law was motivated by intense helicopter traffic over touristic areas like Côte d'Azur (Saint-Tropez and Ramatuelle) where a few wealthy passengers use helicopters to/from heliports located within large cities, creating the largest heliport in Europe irrespective of local population peace and quiet.
Such restrictions are allowed under the legal framework Décret n° 2010-1226 du 20 octobre 2010 portant limitation du trafic des hélicoptères dans les zones à forte densité de population (link).
What can be limited in frequency or forbidden:

Number of helicopter landings and takeoffs per day, month, season, year.
Operations for helicopters which noise performance below a defined criteria.
Time slots, within the day or the week, when engine tests are forbidden.

These restrictions don't affect Medical, Civil Protection or State flights. Aerodrome managers are required to log related helicopter operations.
The order was contested in court by several industries, including helicopter manufacturers and business helicopter pilots, but confirmed. Helicopter traffic is limited, e.g.

For Saint-Tropez to 14 operations per week, 6 per day, on slots 1pm-4pm and 8pm-10am, between May 1 and October 15. Private heliports are further limited.

For Port Grimaud:

Between July 1st and September 15th, total daily traffic limited to 60 movements and prohibition of LDG and TKOF between 1315 and 1545 (local time).


text:
Are there any special considerations involved in designing manual flight controls for supersonic/hypersonic aircraft?
What things, if any, might need to be taken into account in the design of manual flight controls for supersonic or hypersonic aircraft that wouldn’t need to be taken into account with manual controls for subsonic aircraft?

In the transonic region, control forces may become too high for manual control (e.g. controls froze in WW2 planes diving at high speeds), so supersonic aircraft usually have powered controls rather than manual, cable-operated controls.
You also get CoG shift you have to compensate for. This is often done via a Mach trim system or by automatic fuel transfer.


text:
How can the 777X's longer wing reduce vortices?
I saw the following on this web page:

The purpose of the longer wing is to reduce drag caused by vortices, or wake turbulence, that form at an aircraft's wing tips. The less drag, the greater the fuel efficiency, and the more cheaply the aircraft can be operated.

How is this correct? How can a longer wing reduce vortices?

Related, possible duplicates: https://aviation.stackexchange.com/questions/13957/does-the-downwash-created-by-induced-drag-increase-or-decrease-lift https://aviation.stackexchange.com/questions/58277/how-do-endplates-reduce-induced-drag https://aviation.stackexchange.com/questions/32997/is-induced-drag-not-caused-by-tip-vortices

The title and the body of this question don't match. Are you asking about the folding wing, or how wing length affects vorticies?

@RonBeyer: I've edited the question to what I'm pretty confident was what the OP intended to say.

So it can fit into standard size airport gates and taxiways.
One reason the A380 didn't see wider adoption was it exceeded the maximum wingspan that most airports are set up for, requiring airports to build special gates and widen taxiways to accommodate it... which is expensive so very few did that.
Even the airports that did make the A380 mods found that an A380 flight couldn't be rescheduled to any available gate, as they only modified a few berths for the wider wingspan.
By folding the wingtips, the 777-X gets the efficiency of a longer wing and wingtips, but doesn't require a wider than normal gate space and taxiway. It can use any airport that can handle commercial traffic, without requiring expensive changes to the airport.

It sounds like the question is more about the aerodynamics than the space constraints.

Edits to the question title make this answer sound out of place. If you look at the question's edit history, it makes more sense.

Sadly, there have been enough edits to the original question that this no long answers either the title question or the body question.


text:
Is there a difference between altitude and flight level?
I came across a question in Quora asking the difference between altitude and flight level. One of the answers said that anything above the transition altitude is referred to as a flight level. So suppose an aircraft is flying at 35,000ft, it is flight level 350. Is this correct? Can we not refer to altitude as flight level below 18,000ft (the transition altitude)?

Also note: The transition altitude is not necessarily 18,000ft! That's just the US definition.

Yea, sorry my bad!

Yes, altitude is below transition level, flight level is above transition altitude.
Altitude is referencing to actual (local) air pressure, while flight level is against standard pressure 2992 / 1013.
The transition altitude is dependent on the location and are definded so both will not mix up.
When climbing through transition altitude, pilots set their QNH to 2992 - and so no longer need to set it to changes due to local weather.
When decending through transition level they set it back to the local QNH - as they are close enough to ground the real altitude (above ground...) is getting important.

Setting your altimeter to QNH does not give you altitude above ground... For example at my home airport if the altimeter is set correctly it should show about 650 feet sitting in the ground.

Yes, but setting it to QNH shows the "true" altitude - and charts show the ground elevation or minimum altitude... So close to ground this can make a significant difference...

@tsg It does not even show you true altitude AMSL (above mean sea level), unless you are at airport elevation or the temperature happens to be at ISA. See e.g. this article.


text:
What is suction peak?
Can anybody explain to me what suction peak is and does it have anything to do with lift and drag? I've read that suction peak is the point where the coefficient of pressure is the least and is located just behind the leading edge - is this statement true?

The suction peak is the point of lowest pressure on an airfoil, in other words the highest point on a typical pressure or Cp graph of an airfoil. On an airfoil it's typically located just after the leading edge on the upper surface, take a look at this image for instance. In that image you can also see that as you increase the angle of attack of the symmetrical airfoil (i.e. as you increase your lift), the suction peak becomes higher. At your stall angle of attack, the "adverse pressure gradient" becomes too high and the flow separates. An adverse pressure gradient means that the pressure increases.
Note that if you would put a symmetrical airfoil at a negative angle of attack, the pressure peak would appear on the lower side, just behind the leading edge.
Also, the suction peak doesn't have to be just behind the leading edge. It is for many airfoils because the point of highest thickness is quite close to the leading edge, but take a look at a natural laminar flow airfoil for instance. The suction peak is around 65%. Here the goal is to keep the flow laminar as far as possible. An adverse pressure gradient will also favour transition of the flow.

I hope you mean to say the adverse pressure gradient is aft of the suction peak? The suction peak is a decrease in pressure compared to ambient.

Yes of course, the adverse pressure gradient comes after the pressure peak and indeed, the suction peak is the most negative pressure, as I said above.


text:
Does turbulent flow encourage/avert flow separation?
I know that turbulent flow has more energy than laminar flow and hence can sustain adverse pressure gradient for a longer period of time. It basically delays flows separation.
But referring to this source from aviationchief.com, turbulent flow encourages flow separation.
I'm not exactly sure whether statement is true so can anyone give me an explanation behind this concept
This is what I think - Since turbulent flow is known for having skin friction drag, it slows down air after the suction peak. This slow down of air (due to pressure rise and skin friction caused due to turbulent flow) is gonna cause flow separation.

Yeah the dimpled golf ball, right?

yeah that's right

Could you provide a reference to the claim regarding separation? It is true that turbulence increases skin drag, but it also true that the separation point is the one where skin drag disappears. And the turbulent layer is also better att distributing momentum downwards from the free flow to the skin.

Yes it is true that turbulent flow creates skin friction drag.

I haven't heard of turbulent flow encouraging flow separation. Do you mean to say that separated flow usually has turbulent characteristics?

Even I got confused when I came across this statement but I'll provide you with the link where I found this - http://www.aviationchief.com/angle-of-attack.html

It says - "The turbulent flow area increases, encouraging separation of the boundary layer".

It means to say "reversed flow region increases, encouraging separation...".

Sounds like this would be more of an issue as chord increases or velocity decreases, seems to work for a golf ball.

Turbulence delays flow separation. This is why we attempt vortex generators and turbulators in aerodynamic design when local flow separation is an issue.
A. Adverse Pressure Gradient
Fundamental to the generation of subsonic lift, as airflow accelerates over the leading edge, its pressure drops, resulting in net suction. At some point, usually corresponding to the point of re-converging airfoil geometry, the flow begins to decelerate as its pressure rises, eventually equating to the flow pressure on the lower surface at the trailing edge. This process is called pressure recovery, and the increasing pressure profile along the upper surface is called adverse pressure gradient.

Image modified from: https://conself.com/blog/calculate-lift-drag-with-paraview/
B. Laminar Boundary Layer
Within the boundary layer, which is a thin envelope flow close to the airfoil surface, an adverse pressure gradient in the external flow can slow the flow close to the surface (which is already much slower than the external flow due to no-slip condition) to a standstill. At that point, the separation occurs. This is more or less the story with laminar boundary layer where the flow is smooth.

Image ref: https://en.wikipedia.org/wiki/Flow_separation#/media/File:Boundary_layer_separation.svg
C. Turbulent Boundary Layer
Now in the case of a turbulent boundary layer, there is a lot more mixing going on within the boundary layer due to eddies. As a result, there is continued energy injection into the bottom layer flows, which delays the flow separation that would normally occur in a laminar boundary layer.

Image ref: http://www.bakker.org/dartmouth06/engs150/11-bl.pdf

In general a correct answer. But still, there are cases where a laminar boundary layer postpones separation. Every PIK-20 or Janus pilot knows this. With a clean wing, laminar flow provides for a thinner turbulent boundary layer in the region of pressure rise. Once that laminar flow is disturbed by bugs or rain, the stall speed of both those gliders goes up by 20-30%.

The PIK 20 is said to use the Wortmann FX 67 K-170 airfoil, a top lifter, which seems to be right at the edge of the Reynolds number range where top lift works, especially at lower "stall" speeds.  This might make it perfect to study flow separation effects, as well as the benefits of implementing slats.  There may not be an easy answer to the question, as wing types and airspeed vary greatly.


text:
Does the tailplane have a positive/negative camber?
I've read that the tailplane produces negative lift, so does that mean it would function like an inverted wing (has negative camber)? The image below is the best illustration I could come up with.

Also, is negative lift and tail down force the same thing?

I think symmetric airfoil also exists for the tailplane (as they do exist for wing, mainly on aerobatic). It may differ depending on the aircraft, maybe restricting your question to some aircraft type could be useful

Well, when you pull the elevator will sure create a negative camber

The tailplane should produce less lift than the main wing, for pitch stability. That is to say, its lift is negative relative to the wing lift. Its lift need not be wholly negative (pointing down), although it usually is during takeoff and landing.
But yes, as a first approximation, the direction of camber matches the direction of lift.
Also yes, negative lift (at the tail or anywhere else) can be called downforce (at the tail or anywhere else).
Here is Peter's explanation.

Most tailplanes (except for specialist applications, like endurance designs in model airplanes) have a symmetrical airfoil, whether thickened or effectively a flat plate.  Camber of any sort is unusual in tailplanes for full size aircraft -- except as it might be incidentally produced by elevator trim, and in that case, it might be either upward or downward, depending on the trim requirement at any given speed and CG location.

Are the trim tabs cambered/symmetric?

I'd expect trim tabs to continue the same contour as the tailplane in general, so either flat plate sections with rounded/tapered trailing edge, or tapered to match the symmetrical airfoil of the tailplane.

Indeed, the airfoils on many horizontal tail surfaces do have negative camber. This is mostly in combination with powerful flaps on the wing so the tailplane will continue to work with flaps extended, when it needs to develop a relatively high downforce (which is indeed the same as negative lift). The extended wing flaps result in higher wing downwash so the tail "sees" a more negative angle of attack. By cambering the tail airfoil, it will tolerate more negative angles of attack and its minimum lift coefficient is lower than that of a symmetrical airfoil.

I know it is hard to see, but this Do-228 tailplane really does have negative camber (picture source).

The PZL Wilga has a symmetrical tail airfoil but uses an inverted, fixed slat at the leading edge of its elevator (picture source).

The A380 uses negative camber at the root of the tail, too, like most airliners (picture source). To keep isobars aligned with chord lines in a swept wing is also a reason for negative camber at the root.

A few constructions, such as the Zenith CH 701 STOL, have horizontal stabs with non-symmetric, inverted airfoils:


text:
What is a rhumb line?
Can anyone explain what a rhumb line is and how it is different from the great circle route?
This what the internet says:

Rhumb lines are tracks with a constant track direction between two points on a sphere and therefore must be a longer distance than a great circle track.

From pprune.org
What does it mean to have a constant track direction?

Related: Moving dead ahead: rhumb line, great circle or none and How to use a Rhumb Line?

"Must be a longer distance" unless the constant track is 0 or 180, or unless you're on the equator flying 90 or 270.

a rhumb line, rhumb, (/rʌm/) or loxodrome is an arc crossing all meridians of longitude at the same angle, that is, a path with constant bearing as measured relative to true or magnetic north

From: Wikipedia
If continued, all rhumb lines except the ones following a  track of 90 or 270 degrees end up in either the North or South Pole.
If continued, all great circle lines end up back where they started from.
As for the part of the question about constant track direction: track is defined by ICAO as

The projection on the earth’s surface of the path of an aircraft, the direction of which path at any point is usually expressed in degrees from North (true, magnetic or grid).

So, to maintain a constant track would mean that North would always be in the exact same direction from you as you move on. This would eventually take you to one of the poles, unless of course, you were maintaining a track of 90 or 270 degrees.
The picture above shows a constant track of about 75 degrees.

True, but that is pretty much the same thing as in the much debated question of what will happen to the altimeter reading when pressure and/or temperature changes. The track is, as is the altitude, maintained in reference to the reading on the instrument. The turn needed to maintain a track is almost nonexistent untill you are close to the N or S Pole. And anyway, flying straight and level is a very theoretical thing IRL  ;)

@ymb1, a CDI will guide you on orthodrome (great circle) whether GPS-based, VOR-based or even INS-based. But imagine crossing the Atlantic in a post-war DC-6, where the radio instruments would not have any signal and all you had was magnetic compass and a drifting gyroscopic one. You could only fly segments of rhumb line (well, not even that as the wind changes), possibly adjusting the heading at some points to stay closer to the great circle.

In plain English:
Great Circle:
Tie a string tightly around a globe.
It will lie in a perfect circle, known as a "great circle".
This can be done only if the string is stretched tightly around the full circumference; it will fall off if moved slightly in any direction.
The circle can be oriented in any way, but on a globe with markings, obvious positions are along the equator, or along any meridian, passing through both north and south poles.
A great circle passing through any two points represents the shortest distance between them when traveling along the surface.
This is why plane flights from Los Angeles to Hanoi, which is even farther south, pass near the Aleutian Islands in Alaska.
Rhumb Line:
Imagine you are navigating a ship or plane using only a compass.
The simplest and least error-prone method would be to maintain a constant heading throughout the entire trip.
From Portugal, if you continuously sail south-west, you will reach Brazil.
It won't be the shortest or fastest route, but it will be the easiest.
The Mercator Projection map (the "evil one", that makes Greenland look larger than Africa) was designed specifically for this purpose.
Draw a straight line from your start to your destination and that course will have a constant heading all the way.
Without any adjustments along the route, your compass will always indicate the same direction to go.
Every straight line on a Mercator Projection map is a rhumb line.
Except when following the equator (which is both a rhumb line and a great circle), any rhumb line will eventually reach either the north or south pole.

So do pilots follow rhumb line tracks/great circle routes? Both have their own advantage.

@Johnson, generally great circles are the fastest and shortest routes, and navigation aids such as GPS eliminate the advantages of using the simple rhumb-line method.  Pilots will deviate from a strict great circle to make use of high altitude wind (e.g. jet stream) to go faster and to save fuel, and for political reasons (e.g. don't fly over N.Korea).


text:
What does shimmy in nose landing gear mean?
While I was reading a small article on "Reasons for RTO", I came across a situation where the pilots aborted takeoff because of "landing gear shimmy". What does that mean?

the nose gear support strut contains a bearing which allows the nose wheel to swivel like a caster so the plane can turn on the ground. the nose wheel assembly itself is turned either via a hydraulic cylinder or a mechanical linkage connected to pads on the upper tips of the rudder pedals, or in some cases it freely turns on its own during the application of asymmetrical braking to the main gear.
But whatever the details of the nosewheel design are, "shimmy" occurs when the wheel assembly wobbles rapidly back and forth (left-right-left-right) as the plane is rolling down the runway at speed. this is called castering instability and occurs when there is excessive play or looseness in the caster bearing or the support strut or the wheel bearings, or slop in the steering control circuit, or improper inflation of the nose wheel.
The shimmy action sets up strong vibrations in the airframe that the pilot can feel and if strong enough can hear as well. Those vibratory stresses are acting to wiggle loose any bolted connections in the entire nosewheel assembly and can also seek out and activate any fatigue stress raisers present there- in addition to making the airplane difficult to accurately steer.

Like it occasionally happens with worn out shopping carts, right?

@PerlDuck, yes that's it exactly.

I've read that an aircraft performed a RTO because of landing gear shimmy. Why is this so? Is landing gear shimmy that dangerous?

@Johnson You're basically putting 42k of horsepower behind a 300,000 kg triangular / 3-wheeled shopping trolley or pram, and trying to keep it going in a straight line.

@Johnson When you're going very fast in close proximity to the ground, you do not want something applying uncontrolled and unpredictable sideways shoves to the nose of your aircraft.

@Johnson also, after takeoff, you have to land, and unlike takeoff, completing a landing is not negotiable - even if you abort and go around, you still have to try again. It's the old aviation saying: "It's better to be down here wishing you were up there, than to be up there wishing you were down here".

@Johnson Shimmy arises from free play in the movement of some component, which is unwanted and is often implied to be uncontrollable and/or unpredictable. It is this uncontrollable nature that makes it dangerous - you never know if the aircraft will move in the direction you want it to, or how much compensation/counter-steering is required.

So how do pilots know whether they have a shimmy landing gear or not? Do aircrafts have cameras at the belly to indicate that?

@Johnson "How do pilots know?" They can probably feel the vibrations in the controls (at least of a non-complete FBW system) or through the seat-of-their-pants, just as you would in your car. Either way, that's excellent fodder for a whole new question so you can get a complete answer.

Not to mention it'll scare the bejeezus out of you if you're not ready for it in a small GA plane.

it happens in motorcycles too, called a "tankslapper", where the handlebars swivel from full lock left to full lock right twice a second at 70 mph in the fast lane.

@nielsnielsen Ouch! Can't make myself even think about the motorcycle handlebar swivelling like that.

See also https://en.m.wikipedia.org/wiki/Speed_wobble

@PerlDuck Don't you mean how it always happens in any shopping cart?


text:
Why don’t aircraft use run-flat tyres?
Tyre failures on aircraft during takeoff are fairly common, and are usually (though not always) caused by flat or underinflated tyres (via the overdeflection and overheating experienced by tyres with abnormally low internal pressures).  If an aircraft tyre fails during takeoff, this is dangerous in several different ways:

Flying tyre fragments can damage other parts of the aircraft, as well as, potentially, other nearby aircraft or ground personnel.
Wheels with blown tyres are useless for braking, thus greatly increasing the chance of a runway overrun should it become necessary to reject the takeoff.
A tyre failure(s) can greatly degrade directional control, especially on smaller aircraft that only have a few wheels to begin with.
A failed tyre dragging on the runway can easily heat up enough to ignite and burn, dooming the aircraft if the crew then retract the landing gear without knowing that said landing gear is on fire.

Run-flat tyres are designed to remain useable even in the event of a total loss of internal pressure (generally by incorporating an internal weight-bearing skeleton inside the tyre’s air chamber); given the potentially catastrophic consequences of a flat tyre failing during takeoff, why don’t aircraft use run-flat tyres?

Do runflat tires still provide the braking power of a properly inflated tires? Do they reduce drag to the level of an inflated tire? I'm guessing not. Runflat tires are intended to provide a modicum of control and usefulness on a ground vehicle, where braking and drag are not nearly as critical. If that turns out to be the case you have your answer.

Also, I don't think run-flat tires can do 100+mph without disintegrating while they are "flat".

@DJClayworth, the run-flat tires are normally inflated, so in the deflated state they don't need to be as good as inflated, only better than completely deflated.

I think that would be a solution in search of a problem. Other than the Concorde, how many accidents resulting in loss of life (or injury) have there been because of flats? None that I can think of.

This would make an excellent R&D project.  Too many people here afraid of "large pieces flying off".  A woven matrix is the same concept as rebar in concrete (and straw in clay bricks), that is to stop a fracture from breaking completely through and to distribute load.  Tires are one piece of equipment where overdesign may be in order.

The same reasons that run-flat tires aren't more common on cars or trucks are amplified with aircraft.

They're more expensive and can wear out faster. They will also be much heavier. This has to be traded against the cost of an occasional flat tire.
They typically have a maximum speed when flat. Aircraft tires need to operate at much faster speeds than vehicle tires. Many of the examples you provided were made worse by under-inflated tires.
They typically have very limited range when running flat. Again, many of your examples involved under-inflated tires. If they left a regular tire to burst, the same would happen to a run-flat tire.
Many tire failures are due to the tires bursting, not just a simple puncture that makes them go flat. A run-flat design will not help in this case, and may even make the fragments more dangerous.

All of your airliner examples were on pretty old aircraft models, and many included additional factors. Regulations and procedures change over time in response to these sorts of accidents. When properly maintained and operated, aircraft are designed so that a disintegrating tire won't cause serious hazards.

Trading safety for weight and expense needs to be thought about.  Nothing against inflatable tires if they are improved to acceptable safety limits.


text:
What can happen if everyone congregates at the back of a parked plane?
On a recent flight on an A320, after landing, when parked up at the gate, the cabin crew announced something along the lines of "Stairs will be at the front, please do not congregate at the back of the plane due to weight and balance calculations."
Why is this? What can happen?
For me, weight and balance is something I do before a flight relating to take-off performance etc. But obviously that was not the context here.

Related: Can passenger distribution cause a parked airliner to tip?

Loading up the back of a plane can indeed cause it to tip backwards and come to rest on its over-rotation skid. For this reason, some planes actually come with a detachable strut that holds up the tail end during loading. Once fully loaded and properly balanced, the plane's CG is fore of the mains and the strut is removed by the ground crew- hopefully before takeoff!
The critical tipover point (CG moves aft of the mains) will depend on the weight distribution of the cargo and fuel in the plane as it is loading humans. Note also that if the center of lift is fore of the mains, then a sudden wind gust may generate enough lift to rotate the fuselage onto its tail skid. Carnage and mayhem may result.


text:
What is the A320's procedure for dual engine failure at very low altitude?
What is the procedure for a dual engine failure in an Airbus A320 (perhaps a large bird strike, since these engines only tolerate medium birds) at a very low altitude, shortly after takeoff (approx 1,000 ft)? Additionally, will any procedure realistically save an aircraft in such a scenario?

One of the recommendations of the Hudson bay ditching was that there should be a specific checklist for low-altitude dual engine failure that focuses more on flying rather than troubleshooting. I'm curious to see if that's already been made.

@Sanchises Indeed interesting, however the Hudson Bay occurred at a much more forgiving altitude (I think they reached around 3,000 ft).

No procedure would save the aircraft at 1000ft, but the terrain might.

Related

@Sanchises: Hudson Bay ditching?  I don't recall any Airbus being ditched in Hudson Bay - which is in northern Canada.  You may be thinking of the Hudson RIVER, which is a good ways further south :-)  And connected only by both having been discovered by and named for Henry Hudson: https://en.wikipedia.org/wiki/Henry_Hudson

@jamesqf Uh oh, imperialist American spotted ;)

@jamesqf It was a government cover-up. Ditching in the Hudson Bay wasn't quite as impressive so they transported an A320 (actually a repainted B737) to the Hundson river by barge. Alternatively, I didn't pay attention when writing that comment.

Every pilot is taught that in an emergency you fly the plane first and everything else is secondary. Aviate, then communicate. At 1,000 ft there's not going to be much you can do other than figure out how to try to get back down in as few pieces as possible. If you have done your pre-flight planning correctly, long before you line up on the runway you have already thought of what you will do if you find yourself in such a scenario, and discussed it with your FO so you are both on the same page.

... assuming I‘m not the FO! Good answer though.


text:
Are there any truly 'unrecoverable' situations?
I'm starting my quest to obtain a PPL this weekend. So I guess this question is contextualised around light aircraft like the Piper Tomahawk / Cessna, although it certainly would be interesting to get an answer that covers airliners too.
Baring a catastrophic technical failure, such as the wings falling off, a fire, or the horizontal stabiliser failing - is any [pilot caused] situation truly 'unrecoverable'?
I watched this video of a student accidentally starting a spin. Naturally, he looked terrified, but the instructor took over and recovered quickly and easily. This made me wonder, is there anything a student can do that the instructor will not be able to recover from? (obviously smashing the plane into the tarmac on approach isn't really something that can be helped, so I'm only asking about events at altitude here).

The T-tail on the Traumahawk means it could "deep stall", where the elevator gets blanked by the wing and loses effectiveness, which is unusual in a trainer, however it's not unrecoverable, just more difficult. It's also pretty rare.

Think of risk this way Cloud.  Imagine the only vehicle you ever learned to operate was an airplane, and you were presented with learning the drive a car the first time.  You would probably observe the fact that all it takes to get killed in a car is to drift to the left or right say 6 feet.  You'd think, holy crap, I want to go back to my nice safe airplane that lets me wander all over the place without running into things.

There are a dozen scenarios in driving that will get you killed by inattention.  In flying about the only way to kill yourself on a nice day in an airplane with lots of gas aboard is to stall/spin in the circuit.

@JohnK Never mind the fact that it only takes drifting half a dozen feet to get yourself killed in a car; just look at those accident statistics!

a light plane called the AA-1 Yankee is said to have insufficient rudder authority to recover from a spin without very skilled hands on the controls- and its spin recovery characteristics are strongly asymmetric, meaning that in one direction it's supposed to be almost nonrecoverable from a spin. does anyone here know if this is true?

None of the Grummans were certified for spins as they tend to go flat.   Same with the Piper Cherokee.   However, even airplanes that aren't certified for spins are not unrecoverable right away; it just means you can't hold them in a spin because they will go flat after 4 or 5 or 6 rotations.  Just about any modern airplane will recover by itself from a spin entry within the 1st turn or two just be letting go of the controls.

You will see soon enough that the student in this video made a classical error of trying to recover the incipient spin with ailerons. This is exactly what is taught [not to do] in stall/spin recovery lessons. Once you drill it in (likely making a few similar mistakes, with an instructor to help you), you won't be terrified.

One of the more common causes of crash and fatality is a low altitude stall-spin.  A spin, in a modern aircraft, isn't of itself deadly -- with many designs, all you have to do it let go of the controls, and the aircraft will unstall and let you simply fly out of the resulting spiral dive (if you don't wait too long and go past Vne, anyway).
If you get into a stall when low and slow, however, and due to control positions or pilot error initiate a spin, you're likely to hit the ground before you can recover.  Since ground impact in these cases is usually near vertical, nose down, and partially inverted, pilot survival is hardly assured.
Avoiding this kind of fatal mistake is why you practice stalls (with plenty of altitude under you) during flight training -- so you can recognize the conditions that bring on a stall, the aircraft behavior that announces it, and learn how to manage the airplane after the stall occurs to avoid, or intentionally initiate and recover from a spin.
This is (or ought to be) a part of the check ride before flying an unfamiliar type, as well.  Many older designs are less stall friendly than modern (say, post-1960) aircraft -- and if you learned to fly in a Cessna 152, you won't ever have experienced an unintended spin (because the type is difficult to spin even intentionally, and won't do it without large control inputs).  Stall a 1930s light plane at 100 feet, however, and you're likely to fail the "walk away" test.
Another situation that, once in, you can't get out of occurs in mountain flying.  If flying in a canyon under a ceiling (or near your aircraft's ceiling) you may find yourself without room to turn around, but with ground ahead rising into the overcast or faster than your aircraft can climb.  This leads to either "weather" or "Controlled Flight Into Terrain" being the listed cause of the accident, when in both cases it was genuinely pilot error -- this is a situation an aware pilot will avoid.

Unfortunately, my flight school only houses Terrorhawks. You know, the ones they designed to be easy to spin and won't recover unless you input the right controls. Naturally, I'm apprehensive about my lessons, so I'm just trying to figure out how to make sure I never stall or spin in the first place. I certainly don't fancy practicing them. Thanks for the answer by the way.

Flight schools use Tomahawks because it's very difficult to teach spin avoidance and recovery in a trainer that's effectively spin-proof.  You need to learn how to enter and recover from a spin, as well as how to manage a stall in an airplane that will spin.  Otherwise, regardless of requirements to learn to spin and recover to get your license, you'll be in serious trouble the first time something goes badly in an aircraft that isn't spin-proof.

But I would rather just stick to spin-proof planes! I'm a baby! :p

I might suggest that you can learn useful things about stalls, spins, and spin recovery by flying a radio control "advanced trainer" type.  These models fly very much like light aircraft -- but if you crash, you just have to repair or replace a model airframe, nobody gets hurt.

There's no such thing as spin-proof. Don't worry about it unduly, learn the lessons and you'll be fine. Get in the habit of watching your airspeed and balance like a hawk when climbing or on approach, and taking positive action quickly if airspeed gets low or your balance is out. Don't get fixated on it, just make sure to come back to it every few seconds.

Also, ensure your center of mass is where it belongs.  Tail heavy airplanes are insanely prone to stall, moreso since you lose pitch authority as the aircraft noses up and slows, and an aft CG is listed as cause of accidents fairly frequently.  Not just before takeoff; unsecured luggage has killed people.

Of course you're afraid of stalls & spins, @Cloud, you're supposed to be! But, as stated in the 3rd paragraph, that's why they have you practice it - so you know the signs when one begins to happen and you can take the proper corrective action before you end up dead. I'm pretty sure this won't be in your first lesson, either.

Do students still need to demonstrate recovering from a spin? I thought the standard had changed to just have it demonstrated to them.

@CrossRoads In the US, Spins are not even demonstrated... They don't even go full-stall anymore, recovery is initiated at the first sign of a spin (if you get the horn, you've gone too far). I believe Cloud is in the UK though, and I'm not sure how their curriculum works.

@RonBeyer The school tell me that spin and stall recovery is practised at altitude as you approach your first solo. I live in a mountainous area and many Tomahawks have crashed around here.

The UK syllabus for spin training is for incipient spins only (1st quarter turn of the spin). I think Canada is one of the few places that teaches full spins, although if you are training in an airplane not certified for spins, it can also be limited to incipient spins.

@Cloud Light plane crashes in mountainous areas are most often due to either weather (pop-up storm or clouds closing in while flying in a canyon) or CFIT (flying so far up a canyon there isn't room to turn around, but terrain climbing faster than the airplane can climb, for instance).

@ZeissIkon I think the OP may be talking about "attitudes" rather than "situations," but since he did say situations you might add your last comment to your answer. That sounds like an unrecoverable situation to me if you fly into a place where you can neither turn nor climb out of.

@TomMcW Good suggestion.  Editing.

I'm going to make an assumption that solo student flights are in scope here, in which case I would say that flight into IMC (Instrument Meteorological Conditions) would quite often be unrecoverable. Instrument flight is a completely different discipline, without training it takes mere moments to become disoriented and lose control. If you do ever get into IMC glue your eyes on your attitude indicator and do a gentle 180 degree turn to exit while maintaining altitude, do not trust your inner ear!
Another potentially unrecoverable situation would be to fly under steadily lowering cloud until it puts you into a situation where landing safely is not possible. Being trapped between overcast and rising terrain with no landing spots will not end well. Scud-running is just stupid with all the high tension lines and cell towers that have sprung out of the ground.
These are preventable situations. Sometimes people succumb to get-there-itis or peer pressure and convince themselves the clouds are thin, or they're bound to raise in a bit, after all the TAF says so. Good flight planning and sticking to your limits will keep you out of that kind of trouble.

Lowering ceiling isn't the only way to get caught in a bad situation. I've had a training flight that started out quite okay, but the clouds were ominously closing in on us from the sides. (Forecast was for conditions to go from moderate to better; reality was from moderate to worse.) At one point I just decided that it was starting to cut it a bit too close for my comfort, and simply told the instructor that we're heading back to the airport and getting down on the ground. In that case the ceiling was high enough that we could in principle had flown below the clouds, but with little margin.

Well done recognizing the situation and taking positive action @αCVn. I've had inadvertent entry into IMC myself, 8000 ft over the English Channel on my way to France, realized that the TAFs were a superb work of fiction. I have an instrument rating so it wasn't life threatening.

Generally, an airplane being operated within its published limits won't be able to get into a truly unrecoverable situation. (I'm specifically excluding the case where a recovery is theoretically possible, but you don't have enough altitude to execute the procedure before you hit something -- presumably you're already aware that you have less margin for error when you're operating at low altitude.)  However, when you don't operate within approved limits, you can get into truly unfixable trouble.
For example, a flat spin is often unrecoverable.  Most airplanes cannot enter a flat spin while properly loaded, but it's possible for it to happen if the plane's center of gravity is aft of the approved limits.
Depending on your definition of "pilot-caused", ice accumulation  might also qualify.  In addition to simply preventing the wings from generating enough lift, icing can cause a tail plane stall, which may or may not be recoverable.
There are probably others, but the point here is that the plane is designed to behave itself when operated as directed.  If you go outside of those limits, you no longer have any guarantee about what to expect.

Let me go out somewhat on a limb here.

student [...] looked terrified

I'd say that any time you, the pilot, loses composure and/or situational awareness, that can very easily turn a normally recoverable situation into an unrecoverable one.
If you, the pilot, panic, then it's going to become a lot more difficult to fly the airplane safely.
That's one major reason why so much time in training is spent on emergency procedures, including the emergency checklist recitement before take-off, along with the pre-takeoff mentally going through what you'll do if something goes wrong at various points during the take-off and climb-out.
I've had a few pretty good scares myself. One relevant here was when, early during climbout from an uncontrolled airport, one wing probably entered a bit of a downdraft and all of a sudden the airplane was banked something like 20-30 degrees to the side. I quickly wrestled the plane back to about level, almost without thinking, and flew through it, but I don't doubt that if I had lost my composure, it could very easily have led to a crash because there was very little altitude for recovery.
Get-there-itis has already been mentioned, and can be a major factor. Don't be married to the idea of making a take-off, or for that matter to that approach you're flying. A controlled landing, even an outfield landing, is always better than a crash. Aborting on the ground (below decision speed, which isn't much of an issue in small airplanes) and going around on approach, along with more generally buying yourself some margin (by for example climbing or turning), is always an option, even in controlled airspace.

A stall at low altitude would be one unrecoverable situation, but hopefully the instructor would be alert enough to intervene in time to prevent this occurring. Spins are normally recoverable in light aircraft, but this is not always so in fast jets. The F84F Thunderstreak was one example of an early jet fighter in which a spin was considered unrecoverable.

Is it even possible to spin a large jet, i.e. a 737?

Few if any commercial aircraft designed after about 1960 will spin unintentionally, but many can be intentionally spun.  Flaps zero, throttle idle, full aft yoke, full rudder, and she'll spin, at least a bit.  Same way you spin a "spin proof" trainer.

In a Cirrus, spins are also considered "unrecoverable". The Cirrus manual's procedure for "recovering" from a spin is to pull the CAPS lever.

@ZeissIkon  I think "spin proof" means the aircraft will essentially fly itself out of a spin if you just let go, not that you can't make it go in circles if you really try.

@RonBeyer the Cirrus has that ridiculous spin procedure because the FAA allowed them to skip spin testing if the parachute was made the recovery procedure.  The Cirrus has more or less "unknown" spin behavior.  However, most airplanes with "unrecoverable" spin characteristics require full development of the spin to get to the unrecoverable part; several turns at least.  I will bet dollars to donuts that a Cirrus will pop out of a spin if recovery is started within the first turn or two like any other plane.  The idea of pulling a parachute at 6000 ft at the beginning of a spin is nuts to me.

@JohnK Yeah I'm aware that Cirrus petitioned the FAA to avoid spin testing because of the CAPS system. I'm sure that given enough altitude you can pull out even in a Cirrus. It is nuts to me too, however the US does not teach spin recovery training and the Cirrus has a maximum speed (and minimum altitude) at which the CAPS is effective, so if you don't "get it" by the time you exceed CAPS speed, you're screwed. I'm actually disappointed they don't teach spins anymore, and even more disappointed they don't teach full-stall recovery anymore (in the US).

All this talk about Cirrus. I just want to train in one of those now :D What could go wrong?

@RonBeyer IN the UK, IIRC the logic for not requiring spin training for a basic pilot's licence was that there were more accidents and incidents involving intentional spins during training, than from unintentional spins after training. As for stalls, arguably you should never get into a full stall except intentionally, but that assumes that pilots are always aware of what is going on, which is demonstrably not true.

"What could go wrong?" -- most of the same things that could go wrong in a Tomahawk.  Stall at 100 feet after takeoff, and the CAPS won't save you -- no time for the parachute to open before you become part of a pile of shattered composite soaked in avgas.

Regarding some modern aircraft with a spin recovery procedure like deploying parachute, I think one reason is that this lets you have a smaller fin and rudder what means less drag and a higher top speed - good for sales.  Another reason, is it only takes a couple of minutes to demo the parachute whereas it takes quite a while to demonstrate a lot of conventional spin recoveries.  Personally, I prefer a a big fin and rudder and relying on myself to recover from the spin to relying on the execs of a mfr who flim-flammed the FAA.

@RonBeyer The U.S. doesn't teach full-stall recovery? My instructor had me stall and recover literally the first time I ever flew an airplane in the U.S. And my instructor had me demonstrate it several more times on subsequent flights. This has been a few years, but not that many (maybe 6?)

@reirab They do not teach it anymore as per the new airmen certification standards, adopted in 2017. If you've done (or are going to do) a BFR in the US, these will be the standards you are tested to (aka, no full stall). They are not required to teach it, but I think a lot of instructors still do. The point is that the student is supposed to recover at the first sign of a stall (buffet, not the horn), not wait until a stall develops.

@Flynn, they don't 'demo the parachute': you can't fold it back that quickly, and the airframe is generally considered a write-off after parachute landing. But they teach the procedure, of course.

@Zeus There is a repack cost for the parachute, so I think in some situations, the airframe is not a write-off.

@Cloud, I guess this is more for accidental deployment on the ground, or perhaps a procedure required after some time. Normally, the airframe should be assumed a write-off. As it happens, we have a special question about that. I heard this was a significant reason why SR-22 didn't enjoy the best safety record as Cirrus hoped: pilots are reluctant to pull the handle. It took a change in insurance T&C such that now it is effectively the 'get a new airplane' handle :)

The answers above are all very correct, but I do want to emphasize a key philosophical element here.  When you ask is any [pilot caused] situation truly 'unrecoverable', it's important to remember that there is always a point of no return.
There are hundreds of accidents where the pilot took the aircraft out of a situation that was otherwise safe, and ended up crashing the aircraft... sometimes recognizing his/her error just in time to understand what went wrong without time to fix.  Look at recent mishaps with the Thunderbirds, where they entered an aerobatic maneuver at the wrong altitude.  Or look at the Air France flight that went down off Brazil, where they brought the aircraft into a dive that was unrecoverable when recognized.
At any moment, the aircraft could be unrecoverable if your situational awareness is low and you react incorrectly.

Agree 100% with 'a CVn'. Plus there is the oft missed teaching of Spatial Disorientation, is the one killer in aviation that is often 'unrecoverable'. So many accidents have happened under this cause, and have killed a lot of pilots, passengers, as well as complete families. As mentioned above, 'scud running' is dangerous, especially in mountainous terrain, where you may not have enough room to turn around and get out - and crash into a mountainside trying to.  Google 178 seconds to live. There are several ominous videos out there describing this situation that could save your life if one pays attention to the precursors.  The AOPA has several very good instructional videos on this subject as well.


text:
Is there any benefit to folding wings on commercial aircraft?
This video shows NASA researching folding wings, in what looks like a model of an airliner. What would be the benefit to this technology?

Highly related possibly dupe.

@FreeMan partially, but the NASA research allows moving them in flight, which has a lot more effects than just fitting into a smaller gate space.

It would allow an aircraft to have a wingspan of >80 meters yet still be able to be serviced at a standard airport gate.
There are a variety of benefits to longer wingspan, but the major drawback is that commercial passenger airports are designed around that 80 meter box.

It looks like these wings have ends that fold up to become winglets, for drag reduction at high speed, and down to increase wing area, and thus lift, at low speed for takeoff and landing.


text:
Why is there always a fire truck present before refuelling?
Whenever I fly within Europe, I notice there is always a fire truck present for refuelling before takeoff.
Is this standard procedure for every flight in commercial aviation? If yes, is this a global thing or just a European one?

The title is confusing. I think you are meaning "refuelling", not really "before take off", right?

But so my question, is this not frequent on other continents?

@GiacomoCatenazzi Refuelling occurs in a very short windows with budget airlines, so yes, it's usually minutes before takeoff

But before embarkation of most of the passengers (per evacuation rules). Or the presence of firefighter simplify rules of having passengers during refuel?

As far as I know it is not mandated, but ICAO (for which many aviation authorities derive their regulations) states in Annex 14 Section 9.6 Ground servicing of aircraft:

9.6.1 Fire extinguishing equipment suitable for
  at least initial intervention in the event of a fuel
  fire and personnel trained in its use shall be
  readily available during the ground servicing of
  an aircraft, and there shall be a means of
  quickly summoning the rescue and fire fighting service in the event
  of a fire or major fuel spill.

and

9.6.2 When aircraft refuelling operations take
  place while passengers are embarking, on
  board or disembarking, ground equipment shall
  be positioned so as to allow:
   a) the use of a sufficient number of exits for
  expeditious evacuation; and
  b) a ready escape route from each of the exits
  to be used in an emergency

So you've probably been more aware of this if you were actually on the aircraft as it is being refuelled.

I've never seen a fire truck on the ramp at a US airport unless there was an actual emergency, even while an aircraft is being fueled with passengers onboard, so perhaps "readily available" is interpreted differently in different places?

@StephenS, I think the difference in interpretation is in the "fire extinguishing equipment" part of the requirement.  Every time I've seen a plane being fueled in the US, there's been a rather large fire extinguisher nearby.


text:
D.B Cooper incident - What caused the tail to move after a jump?
In the Wiki page on "D.B Cooper", the presumed moment he jumped out of the aircraft is described:

At approximately 8:13 p.m., the aircraft's tail section sustained a sudden upward movement, significant enough to require trimming to bring the plane back to level flight.

This was replicated in a test:

In an experimental re-creation, Scott piloted the aircraft used in the hijacking in the same flight configuration. FBI agents, pushing a 200-pound (91 kg) sled out of the open airstair, were able to reproduce the upward motion of the tail section described by the flight crew at 8:13 p.m

What was the likely cause of this? My initial thought was it was his body striking the tail, but that isn't mentioned and doesn't seem to be what happened in the test.

Upward movement of the tail, i.e. it's a net pitch down for the aircraft?

@JZYL, yes, exactly what you'd get if you suddenly shifted the CG forward.

So happy to see someone asked this question. I was super confused as well reading the wikipedia page but a shifting of the centre of gravity makes sense.

@nhooyr thanks! :)

The "sudden upward tail movement" requires nothing more than a sudden unloading of the tail by removing that 90+ kg of man, parachute, and money.
Even with an aircraft as large as a 727, suddenly removing 90+ kg from the tailcone airstair would result in a pretty noticeable shift in trim requirement, with additional tail downforce required to restore level flight.  This is especially true when flying at the low speed Cooper had demanded (to allow a more-or-less safe jump).

In that case, why does someone getting up to use the rear bathroom and going back to their seat not require re-trimming?

@Zeiss Ikon Also remember that DB's exit happened at the maximum "lever arm" possible in a 727, so the sudden 90+ kg departure would have the maximum effect on weight and balance.

Amazing. Thanks for the insight. I hope they find out what happened some day. I want to know if he survived...

Based on the evidence available (small portions of the money turning up in creeks in wooded parts of southwestern Washington, as well as the crew report of the sudden tail movement) the consensus among those who aren't conspiracy theorists is that he jumped and either failed to open his parachute, or got hung in trees at landing, and died quite promptly.

Dumping small parts of the money into the environment is exactly what I'd do if I was aiming to cover my tracks.

@Harper-ReinstateMonica - that's actually astonishingly astute, and, amazingly, in all the writing about the episode, I've never heard that suggested! Good one!  Adds a new twist to the greatest ever Boy's-Own tale of recent times  :O

This is a great answer, thanks!  Would never have realized this - but it's obvious once outlined.

Cooper was dressed in office attire when the flight left SeaTac -- light clothing and smooth-soled low shoes.  Even if he managed to land unscathed in the heavy woods of southwestern Washington, it's very unlikely he could have walked out of that country dressed like that.  Now, if he had managed to stash proper survival clothing and gear in the airplane, and load up before he jumped, it's possible he could have survived a successful landing -- but in heavy tree cover, you're more likely than not to get hung up high enough off ground to break things when you cut loose.

@Harper-ReinstateMonica true, but given the spread of the finds that's an unlikely scenario.

When you exit an airplane it shakes quite a bit.
On smaller airplanes it's a lot, on larger it's less but still noticable.
Generally it's not something you aim to do, but I think you can make the shake harder by pushing away from the plane more.
Even though his weight is only a small percentage of the airplanes mass, I'm sure you can notice the shake when someone exits the plane.
And then there is the wind deflected on/back in to the airplane.
As you exit the airplane, very often you deflect back wind in to the airplane which also could enhance the shake.
I don't find it a bit odd that there was a shake and upward movement.


text:
Is there any way to determine or estimate where the cloud tops are?
There are various ways to find out or at least estimate where cloud bases are, e.g. ATIS/AWOS/ASOS report them directly, PIREPs may mention them, and you can make a rough estimate based on the temperature/dew point spread.
Is there any way to determine (or estimate) where the tops are, assuming that no PIREPs are available?

https://www.aviationweather.gov/obs/radar/ provides cloud tops for weather with precipitation.
Cloud tops with no precip can be discerned from data provided in the Skew-t log℗ charts that are part of this app http://www.skewtlogpro.com/about-skew-t-log-p-charts/
Where the temperature and dew point on the skew-t charts are converged you can expect clouds. If the convergence extends into the freezing level indicated on the chart you can expect icing.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Are passive or active headsets preferable in an open cockpit?
My first headset used passive noise reduction (i.e. padding) and when I - very quickly - upgraded to an ANR headset, my instructor recommended keeping the passive one "in case you ever fly something with an open cockpit". I can't remember if he gave me an explanation, but he definitely said that passive headsets "worked better" in an open cockpit.
I haven't flown anything open so I've never had the chance to put this to the test, but is it (or was it ever) true?

When I used passive NR headset, my CFI said that it is a good choice as I can hear different engine sounds and relate those to how the airplane is behaving.

@farhan, that doesn't disappear with ANR. They just sound different, they aren't cancelled entirely. No ANR headset has a fast enough processor (or enough data) for that.

@egid I cannot argue as I never used an ANR headset.

First we should see how active noise reduction works. The simplest definition would be:
Active Noise Cancelling uses electric signals to reduce unwanted sounds.

The following picture on Active noise control depicts it in a simple manner:

Then we can see what unwanted sounds ANR can actually reduce. As mentioned in A Guide To Buying An Aviation Headset:

ANR headsets work best at frequencies below about 400-450 Hertz. This
  represents the normal frequency range for speech, and also much
  propeller and exhaust noise is in this region. ANR headsets are
  therefore a significant advantage where intelligibility of
  transmissions is affected by engine and propeller noise.
In general, ANR Headsets offer greater hearing protection in
  high-noise environments but will do little to reduce noise at higher
  frequencies such as wind or airflow noise.

In a flying airplane (even in a slow flight), wind flow would be quite high. This PDF (page 1, paragraph 3) states:

The wind noise spectrum is dominated by the lower frequencies (< 500
  Hz), although at 27 mph when saturation is present, the wind noise
  level can be greater than 60 dB SPL at 8 kHz.

Since passive noise control is sound reduction by noise-isolating materials such as insulation, sound-absorbing tiles, or a muffler rather than a power source, in an open cockpit, passive NR headsets work better than active NR headsets.

I fly a "open cockpit" flex-wing microlight.
I also wear an simple full face helmet that is separate to my headsets.
I fly relatively long flights with many being over 4 hours.
I have also flown for 15.5 hours in a single day in the microlight.
Passive headsets (from differing manufacturers) would often leave a ringing in my ears. After a flight, it would be a minute or two before I could actually hear people speaking to me.
R/T in flight was often difficult to hear.
The difference an ANR headset makes at the end of a flight is night and day.
No ringing in the ears. Can hear people speaking to me fine.
ATC is heard loud and clear.
As for hearing noise the aircraft engine makes, etc, ANR doesn't get rid of all noise, but it does stop you going deaf, so you can at least hear the subtle changes in the engines tone.

I would never use active cancelling for two reasons:

Simplicity. Doing anything that involves more complexity, batteries, wires, etc, is a bad idea. Keep things simple.
You want to hear everything. The more you hear, the better. If something is wrong with the engine, you want to know that immediately, not find it out later when the plane turns into a brick. The point of the headphones is just to turn down the noise to tolerable levels, not eliminate it. When flying, hearing noises is good.

This answer seems to be mostly commentary, and does not really address the question asked.

@NathanG: The question is about preferences, so is actually asking for opinions. If this are backed up by clear reasoning, I wonder why it wouldn't address the question. The answer could be more specific on the particulars of open cockpits, though.

You DON'T want to hear everything.  Long term exposure to high noise levels will damage your hearing.  Importantly, this damage is cumulative (the longer you are exposed to the loud noise, the more likely and more profound the damage to your hearing will be).  And here is the kicker:  The loss of hearing won't be apparent for many years.  Slowly in your later years your hearing will just go bad.  Protect your hearing!  Use ANR headsets when you can.

@SkipMiller On leaving the air force, I was subjected to a discharge medical, I believe it's something to do with lessening the chance of future litigation. My service history was not made available to the various examiners to ensure that there was no bias.  After my hearing test, the examiner asked if I had ever worked on Vulcans, which I had. I asked how he knew.  He showed me the graph of my hearing frequency response which showed a large dip on a part of the chart (memory is dull but I recall it was near the lower end) which apparently is common amongst ex-Vulcan ground handlers.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Was the Cessna 208 developed to meet Fedex's requirements?
I remember hearing or reading somewhere that the Cessna Caravan was developed because Fedex had issued an RFP for a light aircraft to carry cargo on shorter routes and to unimproved airstrips. According to whatever I heard/read, that made it unique as the only mass-produced civilian aircraft to be developed based on a customer's request, at a time when manufacturers built aircraft according to their own opinion of what was needed in the market and then tried to sell them.
Is there any truth in this?

The Wikipedia article says "Working with FedEx, Cessna produced first the Cargomaster, and followed that with the stretched and upgraded Super Cargomaster. The passenger model, the Grand Caravan, was derived from the Super Cargomaster." but is unreferenced.

@RedGrittyBrick Yes, I saw that too but as you said there's no source for it and it doesn't really answer the question of whether Fedex put out a tender for an aircraft or not.

Evidently, the idea for the Cessna 208 existed before FedEx got involved. Cessna started with a widened and stretched 206 fuselage. It was designed to replace the aging De Havilland Canada Otters, Beavers and Beech 18 aircraft in use with bush operators.

Marketing surveys from the late seventies concluded that a modern, large capacity, rugged single engine aircraft for sale could compete successfully...

They completed the first flying prototype and were expecting orders mainly from the target market of bush operators. However, the CEO of FedEx saw it on the cover of Flying Magazine and sensed opportunity. (Whether this is true or not, FedEx was looking for a small aircraft to replace trucks on some routes, and the Caravan came along at just that time.) Cessna made some changes such as adding a cargo pod to create the Cargomaster. The big orders from FedEx helped to drive the popularity of the Caravan.
So FedEx was not the reason for designing the Caravan in the first place, but it was a big part of the success of the design and did influence some aspects of it.